1. Field of the Invention
The field of application of the invention consists of natural draught counter-flow cooling towers of sulphurous fossil fuel-fired power stations provided with flue gas desulphurization installation.
The invention concerns the desulphurized flue gas injection device of these power stations in these cooling towers.
For ecological reasons, basically to stop the damage to forests and the acidification of lakes, and to save their fauna and flora, power stations burning suphurous fuels are forced to take measures so that the flue gas discharged to the atmosphere is low in sulphur content. The measures considered consist in desulphurizing the flue gas at the air preheater outlet.
2. Description of the Prior Art
The current desuphurization processes consist in scrubbing the flue gas with chemical solutions. Patent FR No. 2 534 150 describes such a process. The desulphurized gas is at a relatively low temperature, of the order of 80.degree. C., and is saturated with humidity. This desulphurized gas is too cold to be sent in the normal stacks of power stations and its ascensional force is also reduced by this low temperature, which prevents the fine dispersion of the flue gas in the atmosphere. Consequently, it is desirable to send the flue gas out the natural draught cooling towers to benefit from the powerful ascensional force of the warm air dischrged from these cooling towers, due basically to their large air flow, in spite of the temperature of this air (for example 30.degree. C.) being lower than that of the desulphurized flue gas.
The idea of discharging the flue gas by means of natural draught cooling towers is old as shown in DE No. 347 141. Other references are GB No. 525 702, DE No. 1 601 137, DE No. 2 228 762, DE No. 2 453 488, and, more recently, DE No. 27 38 501 and DE No. 27 52 288 in which the flue gas is desulphurized.
The injection of flue gas in natural draught cooling towers raises some problems, because the flue gas desulphurization is not complete. The initial sulphur content is, for example, of the order of 100 g/m.sup.3, which is lowered to a value of the order of one gram per cubic meter.
The same applies to the other toxic compounds of the flue gas, particularly nitrogen oxides (NO.sub.x). It is desirable to considerably dilute this flue gas in the cooling tower warm air, which has a flow about 10 times higher, to avoid relatively high local concentrations of toxic products at the cooling tower outlet, and to carry the flue gas with the large warm air flow of the cooling tower as a single, nearhomogeneous mass of air. The thorough mixing of the flue gas with the cooling tower air flow will have the effect of increasing the cooling tower draught because of the temperature rise of the mass of mixed air. This matter of mixing appears to have escaped the inventors of the various above-cited patents, in particular of DE No. 2 228 762. Actually the draught increase is desirable not only to reduce the cooling tower dimensions for the same cooling capacity, but also to offset the additional head losses of the cooling tower air flow due to the flue gas pipes of the flue gas injection devices.
Actually, these pipes are very large, for example, for a cooling tower with a diameter of 100 m at the level of the exchange body, above which the flue gas would be injected, the flue gas would be injected by two 7 m diameter pipes.
A characteristic of the desulphurized flue gas is its saturation with water vapour from the wet desulphurization process. The result of this is that any temperaure drop of this flue gas causes the condensation of water vapour in the form of micro-droplets, with a diameter of the order of one micron. Such a cooling of this gas occurs in the pipe which convey it to the inside of the cooling tower. For the cited example (100 m diameter cooling tower, serving a turbo-generator set of 600 MWe), the condensed water flow is of the order of 1 tonne per hour. These water droplets pick up, in part, the sulphur oxides of the flue gas and will therefore be acidified, with their pH being capable of dropping down to 2, for example.
The fine water droplets, of a few microns diameter, are carried far away by the cooling tower air flow and are thus well dispersed in the atmosphere. As against this, the larger water drops, resulting from the coalescence of the fine droplets, or from the separation of the drops from water films formed on colder surfaces, in particular the pipe walls, must be gathered up so that they are not discharged in the atmosphere by the cooling tower and subsequently precipitated close to it, causing damage by their acidity. The means installed to provided the flue gas mixing will therefore have to avoid the presentation of contact surfaces which would be contacted on one side by the relatively hot flue gas (45.degree. to 90.degree. C.) and, on the other side, by the relatively cold (for example 30.degree. C.) cooling tower air flow. Actually, a film of acid water would be formed on the side contacted by the flue gas with separation of water drops of relatively larger diameters, e.g. 0.1 mm-1 mm, and even larger. Mixers as described in patents DE No. 33 27 931, DE No. 30 43 329 or FR No. 2 275 744 are therefore not suitable.